(1) Field of the Invention
This invention relates to shrinkable tubing and, more particularly, to electromagnetic energy absorbing, shrinkable tubing molded of conductive loaded resin-based materials comprising micron conductive powders, micron conductive fibers, or a combination thereof, homogenized within a base resin when molded. This manufacturing process yields a conductive part or material usable within the EMF or electronic spectrum(s).
(2) Description of the Prior Art
Shrinkable tubing is used in the art to protect electrical wiring and/or connectors from electrical shorting as well as to protect from external factors such as moisture, contaminates, and corrosives. Heat shrink tubing is typically a polymer-based material, such as polyolefin, formed by a process of molding the material, irradiating the material to create polymer chain cross-linking, and then stretching the material at a moderate heat such that the material is plastic but not melted. In application, the shrinkable tubing is loosely placed over the wiring or connector and is then heated. This heat treatment energizes the cross-linked polymer bonds and causes the tubing to ‘recover’ to its pre-stretched dimensions. As a result, a tight tube covering is formed over the wiring, connector, or other object.
In addition to environmental protection, another consideration in electronics/electrical systems is electromagnetic interference (EMI). All electronics/electrical systems generate electromagnetic energy due to the movement of electrical charge within the circuit. This electromagnetic energy can be a useful feature, as in the transmission or reception of electromagnetic waves in radio communications. Conversely, electromagnetic energy emanating from a first source can adversely impact the performance of a second source. For example, high energy switching in an automotive ignition system can generate substantial electromagnetic energy emissions that can enter an automotive radio system and result in bad performance. In this case, the ignition emission is considered an electromagnetic interference (EMI) or radio frequency interference (RFI) onto the radio system.
Typically in the art of electrical/electronics systems, attempts are made to shield signal carrying wiring and/or connectors from EMI/RFI through the application of metal shielding structures. For example, a metal mesh sleeve is typically placed around a sensitive signal cable. This sleeve is then connected to ground. Where environmental protection is further desired, shrinkable tubing is additionally placed around the metal mesh sleeve and then heat recovered to complete the shielding and protection application. It is a primary objective of the present invention to provide an improved shrinkable material capable both of environmental protection and of enhanced EMI protection.
Several prior art inventions relate to shrinkable tubing and/or to electromagnetic shielding structures. U.S. Patent Publication US 2002/0037376 A1 to Fenton teaches a heat shrinkable article for shielding against EMI and RFI. U.S. Patent Publication US 2003/0186602 A1 to Millas et al teaches a heat-shrinkable EMI/RF shielding material that utilizes a heat-shrinkable woven fabric coated in a conductive slurry. This invention also teaches constructing the heat-shrinkable woven fabric construction of a mixture of polyolefin and polyester. U.S. Pat. No. 6,005,191 to Tzeng et al teaches a heat-shrinkable jacket for EMI shielding comprising an inner layer of electrically-conductive mesh or fabric covered by an outer layer of a heat-shrinkable polymeric material selected from a group consisting of rubbers, polyolefins, polyamides, polycarbonates, polyesters, and the like.
U.S. Pat. No. 6,064,000 to Kim teaches a heat-shrinkable shielding tube that utilizes an inner layer of metallized fabric that overlaps itself so that the inner longitudinal edge is in direct contact with the outer longitudinal edge and, further, wrapped in an outer layer of a heat shrinkable material. U.S. Pat. No. 5,972,139 to Chu teaches a thermally shrinkable and electromagnetic wave proof tube that utilizes a sleeve of woven soft zinc coated copper wires where the sleeve is placed inside or outside a thermally shrinkable plastic tube. U.S. Pat. No. 4,555,422 to Nakamura et al teaches a heat-shrinkable magnetic shielding article comprising an inner layer of a polymeric material containing a powdery ferrite and an outer layer of a heat shrinkable thermoplastic polymeric material. This invention teaches 10% to 90% ferrite based on the total weight of the inner layer. U.S. Pat. No. 6,410,848 B1 to Shrader et al teaches a side entry heat shrinkable cable jacket that utilizes a conductive fabric inner layer if EMI shielding is desired.